The disclosure relates to electro-discharge machining (EDM) processes, and more particularly to plunge EDM for making a honeycomb extrusion die with reduced slot width variability.
Honeycomb bodies used in catalyst substrate and particulate filtration applications consist of a monolith body having longitudinal, parallel channels defined by longitudinal interconnected webs. The honeycomb bodies are typically made by extruding a plasticized batch material that forms a ceramic material such as cordierite, aluminum titanate or silicon carbide after firing. Extrusion dies used in making the honeycomb bodies have a die body with a discharge end including an array of longitudinal pins defined by interconnected slots. The array of longitudinal pins may include pins having any geometry useful in catalyst substrate and particulate filtration applications, such as rectangular, triangular, or hexagonal. The inlet end of the die body includes feedholes which extend from a base of the die body to the interconnected slots and are used to supply batch material to the slots. To make a honeycomb body using the extrusion die, plasticized batch material is supplied to the feedholes and extruded through the interconnected slots. The batch material extruded through the interconnected slots forms the interconnected webs of the honeycomb body.
Honeycomb extrusion dies are commonly made using plunge EDM processes. In a typical plunge EDM process, a shaped electrode having the desired pin/slot pattern is closely spaced from a workpiece that will become the extrusion die in a bath of dielectric fluid. A voltage is applied across the shaped electrode and the workpiece to cause current to flow between them through the dielectric fluid. In some processes, the shaped electrode is operated under positive polarity, and the workpiece is operated under negative polarity; in other processes, and depending under some circumstances on the composition of the electrode, the current flow may be reversed. The pin/slot pattern is formed in the workpiece by a series of repetitive electrical discharges in the thin gap between the shaped electrode and the workpiece. The electrical discharges generate enough heat to melt the workpiece and transfer the pin/slot pattern of the electrode to the workpiece. As mentioned above, while machining the workpiece, the workpiece is immersed in dielectric fluid, which acts as a conductor for the electrical discharges and at the same time insulates the shaped electrode from the workpiece. The dielectric fluid also serves as a coolant and is used to flush machined chips out of the thin gap between the shaped electrode and the workpiece.
While plunge EDM lends itself well to machining pins and slots of various shapes and sizes within the same extrusion die body, a significant amount of variation in slot width and pin size within a machined extrusion die body has also been observed. Variation in slot width and pin size across the extrusion die translates to variation in web thickness and cell size across the honeycomb body formed with the extrusion die. Significant variation in web thickness and cell size across the honeycomb body can create an undesired thermal distribution profile and/or flow distribution profile in the honeycomb body during subsequent processing or use of the honeycomb body. Thus there is a desire for a plunge EDM process that forms patterns in a workpiece with uniform variation in pattern size across the workpiece, wherein the variation can be controlled to meet manufacturing tolerances.